The present invention relates to a method of forming a printed wiring board, and more particularly to an improved printed wiring board with a compensation scale.
For forming the printed wiring board, it is important to secure an alignment in position between through holes or via holes and a circuit pattern as well as an alignment in position between the through holes or via holes and a solder resist pattern. If no alignment is obtained, any disconnection between the through hole and the circuit pattern is caused, or a solder resist enters into the through hole thereby preventing the soldering with the through hole. The circuit pattern or the through hole may be stripped, which have to be coated with the solder resist.
FIG. 1 is a fragmentary plane view illustrative of a printed wiring board with a circuit pattern having a through hole coated with a solder resist. The printed wiring board has a circuit pattern 10 connected with a circular shaped land 9a which defines a through hole 9. A solder resist 11 is provided which coats the surface of the printed wiring board. The solder resist 11 has an opening 11a which is positioned over a part of the printed circuit board which is different in position from the through hole 9, even the opening 11a should have to be positioned just over the through hole 9. This misalignment between the solder resist film and the pattern film appears, whereby it is impossible to conduct a soldering process in an assembly process for assembling the printed wiring board.
FIG. 2 is a fragmentary plane view illustrative of a printed wiring board with a circuit pattern having a through hole which is not coated with a solder resist. The printed wiring board has a circuit pattern 10 connected with a circular shaped land 9a which defines a through hole 9. A solder resist 11 is provided which coats the surface of the printed wiring board. The solder resist 11 has an opening 11a which is positioned over the through hole 9. This perfect alignment between the solder resist film and the pattern film appears, whereby it is possible to conduct a soldering process in an assembly process for assembling the printed wiring board.
Accordingly, the perfect alignment of the through hole or the via hole and the circuit pattern and the solder resist is essential for forming the printed wiring board.
The circuit pattern is formed on the printed wiring board as follows. A photo-resist is coated over a copper foil. An exposure mask comprising a pattern film which has a circuit pattern is so positioned as to align a reference mark formed on the exposure mask to a through hole or a via hole. An exposure and a development are carried out to form an etching resist over the copper foil, so that a wet etching process is carried out by use of the etching resist to form a circuit pattern.
The solder resist is formed over the printed wiring board as follows. Similarly to the formation of the circuit pattern, a photo-resist is provided which coats the printed wiring board. Subsequently, a solder resist film as an exposure mask for the purpose of forming a solder resist is positioned so as to align a reference mark formed on the solder resist film to a mark formed on the printed wiring board for carrying out an exposure process and a subsequent development process thereby forming a solder resist pattern on the printed wiring board.
The alignments of the mask films for forming the circuit pattern and the solder resist are carried out with reference to the reference marks formed on the films by human eyes. However, the recent increase in high density of the printed wiring board and the high accuracy thereof make it difficult to obtain a perfect alignment with the human eyes.
In prior art techniques, there is a further problem in misalignment between the circuit pattern or the solder resist and the through hole or via hole. The printed wiring board receives a mechanical stress during processes for forming the same and also receive a thermal stress, whereby the printed wiring board may be strained. This strain causes misalignment of the mask film for the exposure process. In order to prevent the misalignment, it is required to compensate the exposure mask film for the circuit pattern and the solder resist pattern in accordance with the strain of the printed wiring board due to the received mechanical and thermal stresses. If the mask film receives a mechanical or thermal stress and then is strained, it is also required to the exposure mask film in accordance with the strain of the exposure mask film.
FIG. 3 is a plane view illustrative of a printed wiring board which has reference holes for calculating an amount of strain caused by received mechanical and/or thermal stresses. The printed wiring board 1 has four reference holes 8 at intermediate positions of every four sides of the board 1, so that a distance between two of the reference holes 8 positioned in opposite sides is measured to calculate the strain. The exposure mask is formed with compensation made in accordance with the calculated amount of strain. The mask film is used for the exposure process to form the circuit pattern or the solder resist pattern.
The compensation is thus made on the basis of a variation in distance between the reference holes formed on the printed wiring board. In this case, it is required to measure every times variations in distances between the reference holes of the printed wiring board for calculating the amount of strain of the printed wiring board and calculating a necessary amount of the compensation in alignment. Those processes increase a total number of fabrication processes of the printed wiring board. Those increase in the number of the manufacturing processes also results in increase in the manufacturing cost. In order to prevent the increase in the number of the manufacturing processes and the increase in the manufacturing cost, it is required to hesitate to carry out the measurement of the distance between the reference holes unless the exposure film alignment is unobtainable.
A technique for improvement in measurement of the strain of the printed wiring board on off-line is disclosed in Japanese laid-open patent publication No. 60-249385. Main alignment marks are provided on intermediate positions on each four sides of the printed wiring board and subordinate alignment marks are provided on four comers of the printed wiring board.
This technique is effective to align the pattern film and the solder resist film, but ineffective to align between the through hole or via hole and the circuit pattern or the solder resist pattern, for which reason the above technique is insufficient for responding to the requirements for compensations to the strain caused by the mechanical and/or thermal stress.
As a further conventional technique, it is disclosed in Japanese laid-open patent publication No. 6-177505 that positioning pilot holes are formed at the same time when the through holes are formed in the printed wiring board.
As a further more conventional technique, it is disclosed in Japanese laid-open patent publication No. 60-249385 that both main scales and subordinate scales as alignment marks are formed on a film over the printed wiring board. Since both main scales and subordinate scales as alignment marks are formed on the film, then it is impossible to make a highly accurate compensation in alignment between a through hole or a via hole and a circuit pattern film or a solder resist film on the basis of a strain caused by mechanical and/or thermal stresses that at least any one of the printed wiring board and either the circuit pattern film or the solder resist film has received.
The through holes or via holes have been formed before the circuit pattern or the solder resist pattern is formed with reference to the positions of the through holes or via holes with compensation to strain of at least any one of the printed wiring board and the exposure film.
In the above circumstances, it had been required to develop a novel method of forming a printed wiring board free from the above problem.